PipeWriter.UnflushedBytes long property wraps to negative numbers after int.MaxValue (#115978)

* PipeWriter.UnflushedBytes long property wraps to negative numbers after int.MaxValue

* forgotten

* play the test is memory abundant envs

* fix remarks round 1

* delete trailing line

* change test name

Author: pedrobsaila

src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeWriter.cs

@@ -20,7 +20,7 @@ internal sealed class StreamPipeWriter : PipeWriter
         private BufferSegment? _tail;
         private Memory<byte> _tailMemory;
         private int _tailBytesBuffered;
-        private int _bytesBuffered;
+        private long _bytesBuffered;
 
         private readonly MemoryPool<byte>? _pool;
         private readonly int _maxPooledBufferSize;

src/libraries/System.IO.Pipelines/tests/PipeWriterTests.cs

@@ -360,5 +360,20 @@ public async Task GetMemoryFlushWithACompletedReaderNoops()
             Assert.Equal(0, pool.CurrentlyRentedBlocks);
             Assert.Equal(0, Pipe.Writer.UnflushedBytes);
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotBrowser), nameof(PlatformDetection.Is64BitProcess))]
+        public void UnflushedBytes_HandlesLargeValues()
+        {
+            PipeWriter writer = PipeWriter.Create(Stream.Null);
+            int bufferSize = 10000;
+
+            while (writer.UnflushedBytes >= 0 && writer.UnflushedBytes <= int.MaxValue)
+            {
+                writer.GetMemory(bufferSize);
+                writer.Advance(bufferSize);
+            }
+
+            Assert.Equal(2147490000, writer.UnflushedBytes);
+        }
     }
 }